Frequency converted dimming signal generation

ABSTRACT

There is provided a lighting control circuit comprising a duty cycle detection circuit, an averaging circuit, a waveform generator and a comparator circuit. The duty cycle detection circuit generates a first periodic waveform having a duty cycle and frequency corresponding to an input waveform duty cycle and frequency. The averaging circuit generates a first signal having a voltage level corresponding to the duty cycle of the first periodic waveform. The waveform generator outputs a second periodic waveform having a frequency different from the input waveform frequency. The comparator circuit compares the second periodic waveform with the first signal to generate an output waveform having a duty cycle corresponding to the input waveform duty cycle and a frequency corresponding to the frequency of the second periodic waveform. Also, there are provided methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 61/022,886, filed Jan. 23, 2008, the entirety of which is incorporated herein by reference.

This application claims the benefit of U.S. Provisional Patent Application No. 61/039,926, filed Mar. 27, 2008, the entirety of which is incorporated herein by reference.

RELATED APPLICATION

The present application is related to U.S. patent application Ser. No. 12/328,115, entitled “DIMMING SIGNAL GENERATION AND METHODS OF GENERATING DIMMING SIGNALS” filed Dec. 4, 2008 (now U.S. Patent Publication No. 2009/0184662), the disclosure of which is incorporated herein as if set forth in its entirety.

FIELD OF THE INVENTION(S)

The present inventive subject matter relates to lighting devices and more particularly to power control for light emitting devices in the presence of a dimming signal where pulse width is a reflection of dimming level.

BACKGROUND OF THE INVENTION(S)

Many control circuits for lighting utilize phase cut dimming. In phase cut dimming, the leading or trailing edge of the line voltage is manipulated to reduce the RMS voltage provided to the light. When used with incandescent lamps, this reduction in RMS voltage results in a corresponding reduction in current and, therefore, a reduction in power consumption and light output. As the RMS voltage decreases, the light output from the incandescent lamp decreases.

An example of a cycle of a fall wave rectified AC signal is provided in FIG. 1A, a cycle of a phase cut rectified AC waveform is illustrated in FIG. 1B and a cycle of a reverse phase cut AC waveform is illustrated in FIG. 1C. As seen in FIGS. 1A through 1C, when phase cut dimming is utilized, the duty cycle of the resulting rectified waveform is changed. This change in duty cycle, if sufficiently large, is noticeable as a decrease in light output from an incandescent lamp. The “off” time does not result in flickering of the incandescent lamp because the filament of an incandescent lamp has some thermal inertia and will remain at a sufficient temperature to emit light even during the “off” time when no current flows through the filament.

Recently, solid state lighting systems have been developed that provide light for general illumination. These solid state lighting systems utilize light emitting diodes or other solid state light sources that are coupled to a power supply that receives the AC line voltage and converts that voltage to a voltage and/or current suitable for driving the solid state light emitters. Typical power supplies for light emitting diode light sources include linear current regulated supplies and/or pulse width modulated current and/or voltage regulated supplies.

Many different techniques have been described for driving solid state light sources in many different applications, including, for example, those described in U.S. Pat. No. 3,755,697 to Miller, U.S. Pat. No. 5,345,167 to Hasegawa et al, U.S. Pat. No. 5,736,881 to Ortiz, U.S. Pat. No. 6,150,771 to Perry, U.S. Pat. No. 6,329,760 to Bebenroth, U.S. Pat. No. 6,873,203 to Latham, II et al, U.S. Pat. No. 5,151,679 to Dimmick, U.S. Pat. No. 4,717,868 to Peterson, U.S. Pat. No. 5,175,528 to Choi et al, U.S. Pat. No. 3,787,752 to Delay, U.S. Pat. No. 5,844,377 to Anderson et al, U.S. Pat. No. 6,285,139 to Ghanem, U.S. Pat. No. 6,161,910 to Reisenauer et al, U.S. Pat. No. 4,090,189 to Fisler, U.S. Pat. No. 6,636,003 to Rahm et al, U.S. Pat. No. 7,071,762 to Xu et al, U.S. Pat. No. 6,400,101 to Biebl et al, U.S. Pat. No. 6,586,890 to Min et al, U.S. Pat. No. 6,222,172 to Fossum et al, U.S. Pat. No. 5,912,568 to Kiley, U.S. Pat. No. 6,836,081 to Swanson et al, U.S. Pat. No. 6,987,787 to Mick, U.S. Pat. No. 7,119,498 to Baldwin et al, U.S. Pat. No. 6,747,420 to Barth et al, U.S. Pat. No. 6,808,287 to Lebens et al, U.S. Pat. No. 6,841,947 to Berg-johansen, U.S. Pat. No. 7,202,608 to Robinson et al, U.S. Pat. No. 6,995,518, U.S. Pat. No. 6,724,376, U.S. Pat. No. 7,180,487 to Kamikawa et al, U.S. Pat. No. 6,614,358 to Hutchison et al, U.S. Pat. No. 6,362,578 to Swanson et al, U.S. Pat. No. 5,661,645 to Hochstein, U.S. Pat. No. 6,528,954 to Lys et al, U.S. Pat. No. 6,340,868 to Lys et al, U.S. Pat. No. 7,038,399 to Lys et al, U.S. Pat. No. 6,577,072 to Saito et al, and U.S. Pat. No. 6,388,393 to Illingworth.

In the general illumination application of solid state light sources, one desirable characteristic is to be compatible with existing dimming techniques. In particular, dimming that is based on varying the duty cycle of the line voltage may present several challenges in power supply design for solid state lighting. Unlike incandescent lamps, LEDs typically have very rapid response times to changes in current. This rapid response of LEDs may, in combination with conventional dimming circuits, present difficulties in driving LEDs.

For example, one way to reduce the light output in response to the phase cut AC signal is to utilize the pulse width of the incoming phase cut AC line signal to directly control the dimming of the LEDs. The 120 Hz signal of the full-wave rectified AC line signal would have a pulse width the same as the input AC signal. This technique limits the ability to dim the LEDs to levels below where there is insufficient input power to energize the power supply. Also, at narrow pulse width of the AC signal, the output of the LEDs can appear to flicker, even at the 120 Hz frequency. This problem may be exacerbated in 50 Hz systems as the full wave rectified frequency of the AC line is only 100 Hz.

Furthermore, variation in the input signal may affect the ability to detect the presence of a phase cut dimmer or may make detection unreliable. For example, in systems that detect the presence of a phase cut dimmer based on detection of the leading edge of the phase cut AC input, if a reverse-phase cut dimmer is used, the dimming is never detected. Likewise, many residential dimmers have substantial variation in pulse width even without changing the setting of a dimmer. If a power supply detects the presence of dimming based on a threshold pulse width, the power supply could detect the presence of dimming on one cycle and not on another as a result of this the variation in pulse width.

A further issue relates to AC dimmers providing some phase cut even at “full on.” If the LEDs are directly controlled by the AC pulse width, then the LEDs may never reach full output but will dim the output based on the pulse width of the “full on” signal. This can result in a large dimming of output. For example, an incandescent lamp might see a 5% reduction in power when the pulse width is decreased 20%. Many incandescent dimmers have a 20% cut in pulse width at full on, even though the RMS voltage is only reduced 5%. While this would result in a 5% decrease in output of an incandescent, it results in a 20% decrease in output if the phase cut signal is used to directly control the LEDs.

SUMMARY OF THE INVENTION(S)

The frequency converted dimming circuits described herein may overcome one or more of the problems associated with dimming directly from a phase cut input AC line. Embodiments of the present inventive subject matter may be particularly well suited to controlling a drive circuit for solid state lighting devices, such as LEDs. In particular, an input waveform with an input frequency and duty cycle are converted to an output waveform with an output frequency with a duty cycle that is based on the input duty cycle. In some embodiments, the output frequency is greater than the input frequency. For example, when the input waveform is a phase cut AC line input, the output frequency may be greater than the input frequency so as to reduce or eliminate the perception of flicker in a lighting device that is dimmed by the phase cut of the AC line input. By increasing the switching frequency, the flicker becomes undetectable to the human eye, but the integrated value of duty-cycle of the light remains, effectively dimming the LEDs.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1C are examples of a cycle of a full wave rectified AC line signal with and without phase cut dimming.

FIG. 2 is a block diagram of a lighting device incorporating duty cycle detection and frequency conversion according to some embodiments of the present inventive subject matter.

FIG. 3 is a block diagram of a lighting device suitable for use in an AC phase cut dimming system according to some embodiments of the present inventive subject matter.

FIG. 4 is a block diagram of a duty cycle detection and frequency conversion circuit according to some embodiments of the present inventive subject matter.

FIGS. 5A and 5B are waveform diagrams illustrating alternative duty cycle detection techniques suitable for use in duty cycle detection circuits according to some embodiments of the present inventive subject matter.

FIGS. 6A and 6B are timing diagrams illustrating operation of averaging, waveform generator and comparator circuits according to some embodiments of the present inventive subject matter.

FIG. 7 is a block diagram of a duty cycle detection and frequency conversion circuit according to further embodiments of the present inventive subject matter.

FIG. 8 is a block diagram of a duty cycle detection and frequency conversion circuit according to further embodiments of the present inventive subject matter.

FIG. 9 is a circuit diagram of a duty cycle detection and frequency conversion circuit utilizing symmetric pulse width detection according to some embodiments of the present inventive subject matter.

FIG. 10 is a circuit diagram of a duty cycle detection and frequency conversion circuit utilizing asymmetric pulse width detection according to further embodiments of the present inventive subject matter.

FIG. 11 is a circuit diagram of a duty cycle detection and frequency conversion circuit according to further embodiments of the present inventive subject matter.

FIG. 12 is a circuit diagram of a system as illustrated in FIG. 2 according to some embodiments of the present inventive subject matter.

FIG. 13 is a flowchart illustration of operations of some embodiments of the present inventive subject matter.

FIG. 14 is a flowchart illustration of operations according to further embodiments of the present inventive subject matter.

FIGS. 15A through 15E are representative examples of waveform shapes for the waveform generator according to the present inventive subject matter.

FIGS. 16A-16F are circuit diagrams depicting an embodiment of a circuit according to the present inventive subject matter.

DETAILED DESCRIPTION OF THE INVENTION(S)

The present inventive subject matter now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the inventive subject matter are shown. However, this inventive subject matter should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive subject matter to those skilled in the art. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the inventive subject matter. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

As noted above, the various aspects of the present inventive subject matter include various combinations of electronic components (transformers, switches, diodes, capacitors, transistors, etc.). Persons skilled in the art are familiar with and have access to a wide variety of such components, and any of such components can be used in making the devices according to the present inventive subject matter. In addition, persons skilled in the art are able to select suitable components from among the various choices based on requirements of the loads and the selection of other components in the circuitry. Any of the circuits described herein (and/or any portions of such circuits) can be provided in the form of (1) one or more discrete components, (2) one or more integrated circuits, or (3) combinations of one or more discrete components and one or more integrated circuits.

A statement herein that two components in a device are “electrically connected,” means that there are no components electrically between the components that materially affect the function or functions provided by the device. For example, two components can be referred to as being electrically connected, even though they may have a small resistor between them which does not materially affect the function or functions provided by the device (indeed, a wire connecting two components can be thought of as a small resistor); likewise, two components can be referred to as being electrically connected, even though they may have an additional electrical component between them which allows the device to perform an additional function, while not materially affecting the function or functions provided by a device which is identical except for not including the additional component; similarly, two components which are directly connected to each other, or which are directly connected to opposite ends of a wire or a trace on a circuit board or another medium, are electrically connected.

Although the terms “first”, “second”, etc. may be used herein to describe various elements, components, regions, layers, sections and/or parameters, these elements, components, regions, layers, sections and/or parameters should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present inventive subject matter.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this inventive subject matter belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

FIG. 2 is a block diagram of a lighting device 10 incorporating embodiments of the present inventive subject matter. As seen in FIG. 2, the lighting device 10 includes a driver circuit 20 and one or more LEDs 22. The LED driver circuit 20 is responsive to a duty cycle detection and frequency conversion circuit 24. The duty cycle detection and frequency conversion circuit 24 receives a variable duty cycle input signal of a first frequency and outputs a fixed amplitude signal having a second frequency different from the first frequency and with a duty cycle that is dependent on the duty cycle of the variable duty cycle input signal.

The duty cycle of the output waveform of the duty cycle detection and frequency conversion circuit 24 may be substantially the same as the duty cycle of the input signal or it may differ according to a predefined relationship. For example, the duty cycle of the output waveform may have a linear or non-linear relationship to the duty cycle of the input signal. Likewise, the duty cycle of the output waveform will typically not track the duty cycle of the input waveform on a cycle by cycle basis. Such may be beneficial if substantial variations may occur in the duty cycle of the variable duty cycle waveform, for example as may occur in the output of a conventional AC phase cut dimmer even without changing the setting of the dimmer. Therefore, the output waveform of the duty cycle detection and frequency conversion circuit 24 will, in some embodiments, have a duty cycle that is related to a smoothed or average duty cycle of the input signal. This smoothing or averaging of the input duty cycle may reduce the likelihood that unintended variations in the duty cycle of the input waveform will result in undesirable changes in intensity of the light output by the lighting device 10 while still allowing for changes in the dimming level. Further details on the operation of duty cycle detection and frequency conversion circuits according to some embodiments of the present inventive subject matter are provided below.

The driver circuit 20 may be any suitable driver circuit capable of responding to a pulse width modulated input that reflects the level of dimming of the LEDs 22. The particular configuration of the LED driver circuit 20 will depend on the application of the lighting device 10. For example, the driver circuit may be a boost or buck power supply. Likewise, the LED driver circuit 20 may be a constant current or constant voltage pulse width modulated power supply. For example, the LED driver circuit may be as described in U.S. Pat. No. 7,071,762. Alternatively, the LED driver circuit 20 may be a driver circuit using linear regulation, such as described in U.S. Pat. No. 7,038,399 and in U.S. Patent Application No. 60/844,325, filed on Sep. 13, 2006, entitled “BOOST/FLYBACK POWER SUPPLY TOPOLOGY WITH LOW SIDE MOSFET CURRENT CONTROL” (inventor: Peter Jay Myers), and U.S. patent application Ser. No. 11/854,744, filed Sep. 13, 2007 (now U.S. Patent Publication No. 2008/0088248), entitled “Circuitry for Supplying Electrical Power to Loads,” the disclosures of which are incorporated herein by reference as if set forth in their entirety. The particular configuration of the LED driver circuit 20 will depend on the application of the lighting device 10.

FIG. 3 illustrates further embodiments of the present inventive subject matter where a lighting device 30 is powered from an AC line input where the duty cycle of the AC line input varies. Such an input may, for example, be provided by utilizing a phase cut dimmer to control the duty cycle of the AC line input. The lighting device 30 includes one or more LEDs 22, an LED driver circuit 40, a power supply 42 and a duty cycle detection and frequency conversion circuit 44. The power supply 42 receives an AC line input and provides power to the LED driver circuit 40 and the duty cycle detection and frequency conversion circuit 44. The power supply 42 may be any suitable power supply including, for example, buck or boost power supplies as described in U.S. patent application Ser. No. 11/854,744 (now U.S. Patent Publication No. 2008/0088248). Also, the LED driver circuit 40 may be any suitable LED driver circuit capable of varying the intensity of the output of the LEDs 22 in response to a fixed amplitude signal of variable duty cycle. The particular configurations of the LED driver circuit 40 and/or the power supply 42 will depend on the application of the lighting device 30.

As is further seen in FIG. 3, the duty cycle detection and frequency conversion circuit 44 receives the rectified AC input from the power supply 42 and detects the duty cycle of the rectified AC input. By detecting duty cycle rather than RMS voltage, the duty cycle detection and frequency conversion circuit 44 may be less sensitive to variations in the AC input voltage (for example, if duty cycle were estimated by instead tracking RMS voltage, an AC line voltage drop from 120 VAC to 108 VAC would bring about an incorrect reduction in the estimated duty cycle, i.e., variations in input voltage may be misinterpreted as changes in duty cycle and result in an undesired dimming of the light output). In contrast, by detecting duty cycle rather than RMS voltage, variations in the voltage level will only be reflected as small variations in the detected duty cycle that result from changes in slew rate for the voltage to reach the differing voltage levels.

In addition to generating a frequency converted fixed amplitude waveform having a duty cycle that is related to the duty cycle of the input wave form, the duty cycle detection and frequency conversion circuits 24 and/or 44 of FIGS. 2 and/or 3 may also detect when the duty cycle of the input waveform has fallen below a minimum threshold and output a shutdown signal. The shutdown signal may be provided to the power supply 42 and/or the LED driver circuit 20 or 40. In some embodiments, the shutdown signal may be provided to turn off the LEDs at a time before the input power to the lighting device 10 or 30 reaches a level that is below a minimum operating level of the lighting device 10 or 30. Alternatively or additionally, the shutdown signal may be provided to turn off the LEDs at a time before the power drawn by the lighting device 10 or 30 reaches a level that is below a minimum operating power for a dimmer control device, such as a triac dimmer or other phase cut dimmer.

FIG. 4 illustrates functional blocks for a duty cycle detection and frequency conversion circuit 100 according to some embodiments of the present inventive subject matter. The duty cycle detection and frequency conversion circuit 100 utilizes pulse width detection of a variable duty cycle waveform to provide a duty cycle detection circuit 110. The output of the duty cycle detection circuit 110 is a fixed amplitude waveform with a duty cycle corresponding to (i.e., based on, but not necessarily differing from) the duty cycle of the input waveform (e.g., depending on the embodiment according to the present inventive subject matter, similar to, slightly less than, related to or inversely related to the duty cycle of the input waveform). The expression “related to” encompasses relationships where the variance of the duty cycle of the output of the duty cycle detection circuit is proportional to the variance of the duty cycle of the input waveform (i.e., there is a linear relationship between the two), or where there is no linear relationship and if the duty cycle of the input waveform increases, the duty cycle of the output of the duty cycle detection circuit also increases, and vice-versa (i.e., if the duty cycle of the input waveform decreases, the duty cycle of the output of the duty cycle detection circuit also decreases); conversely, the expression “inversely related to” encompasses relationships where the variance of the duty cycle of the output of the duty cycle detection circuit is inversely proportional to the variance of the duty cycle of the input waveform, or where there is no linear inverse relationship and if the duty cycle of the input waveform decreases, the duty cycle of the output of the duty cycle detection circuit increases, and vice-versa.

The output of the duty cycle detection circuit is provided to an averaging circuit 120 that creates an average value of the output of the duty cycle detection circuit. In some embodiments, the average value is reflected as a voltage level. A high frequency waveform is provided by the waveform generator 130. The waveform generator 130 may generate a triangle, sawtooth or other periodic waveform. In some embodiments, the frequency of the waveform output by the waveform generator 130 is greater than 200 Hz, and in particular embodiments, the frequency is about 300 Hz (or higher). The shape of the waveform may be selected to provide the desired relationship between the duty cycle of the input signal and the duty cycle of the frequency converted pulse width modulated (PWM) output. The output of the waveform generator 130 and the output of the averaging circuit 120 are compared by the comparator 140 to generate a periodic waveform with the frequency of the output of the waveform generator 130 and a duty cycle based on the output of the averaging circuit 120.

Operation of a first embodiment of a duty cycle detection and frequency conversion circuit 100 will now be described with reference to the waveform diagrams of FIGS. 5A, 5B, 6A and 6B. In particular, FIGS. 5A and 5B illustrate duty cycle detection utilizing a symmetric threshold (FIG. 5A) and alternative embodiments utilizing asymmetric thresholds (FIG. 5B). In either case, the voltage level of the input waveform is compared to a threshold voltage.

In the symmetric example (FIG. 5A), if the input voltage is above the threshold voltage, the output of the duty cycle detection circuit 110 is set to a first voltage level (in this embodiment, 10 volts) and if the input voltage level is below the threshold voltage, the output of the duty cycle detection circuit 110 is set to a second voltage level (in this embodiment, 0 volts, i.e., ground). Thus, the output of the duty cycle detection circuit 110 is a square wave that transitions between the first voltage level and the second voltage level (e.g., 10 V and ground). The first and second voltage levels may be any suitable voltage levels and may be selected based upon the particular averaging circuit utilized.

In the asymmetric example (FIG. 5B), if the input voltage is above a first threshold, the output of the duty cycle detection circuit 110 is set to a first voltage level and remains at that voltage level until the input voltage level falls below a second threshold voltage at which time the output of the duty cycle detection circuit 110 is set to a second voltage level. Thus, in the asymmetric example, the output of the duty cycle detection circuit 110 is also a square wave that transitions between the first voltage level and the second voltage level (e.g., 10 V and ground). As described above, the first and second voltage levels may be any suitable voltage levels and may be selected based upon the particular averaging circuit utilized. The asymmetric detection may allow for compensation for variations in the input waveform. For example, if the leading or trailing edges of a phase cut waveform intermittently include a section with a shallow slope followed or preceded by a section with a steep slope, the separate thresholds could be set to align with the section of steep slope so as to avoid minor variations in duty cycle being amplified by the shallow slope portions of the waveform.

FIG. 6A illustrates operation of the averaging circuit 120. As seen in FIG. 6A, the averaging circuit 120 averages a fixed amplitude periodic waveform with varying duty cycle to provide an averaged square wave signal having a voltage that (in this embodiment) represents the duty cycle of the input waveform. The level of averaging may be set to smooth out variations in the duty cycle of the input signal.

This embodiment thus provides an averaged square wave signal which is related to the duty cycle of the input voltage. For example, if (1) the duty cycle of the input voltage is 60%, (2) the duty cycle of the output of the duty cycle detection circuit is 55%, (3) the first voltage level is 10 V and (4) the second voltage level is 0 V, the voltage of the averaged square wave signal would be about 5.5 V. Alternatively, in other embodiments according to the present inventive subject matter, the averaged square wave signal can instead be inversely related to the duty cycle of the input voltage. For example, if the first voltage level is ground and the second voltage level is 10 V, the inverse relationship would be provided (to illustrate, for such an embodiment, if (1) the duty cycle of the input voltage is 85% and the threshold voltage is 0 V (e.g., zero cross detection AC sensing is employed), the duty cycle of the output of the duty cycle detection circuit would be 15% (i.e., for 85% of the time, the voltage level would be ground, which is the first voltage level, and for 15% of the time, the voltage level would be 10 V, which is the second voltage level), such that the voltage of the averaged square wave signal would be about 1.5 V (whereas is the duty cycle of the input voltage were 10%, the voltage of the averaged square wave signal would be about 9 V).

It should also be noted that it is not necessary for either of the first voltage level or the second voltage level to be zero. For instance, if (1) the duty cycle of the input voltage is 80%, (2) the duty cycle of the output of the duty cycle detection circuit is 70%, (3) the first voltage level is 20 V and (4) the second voltage level is 10 V, the voltage of the averaged square wave signal would be about 17 V (i.e., the voltage of the averaged square wave signal would be between 10 V and 20 V, and would vary within that range proportionally to the duty cycle of the output of the duty cycle detection circuit.

FIG. 6B illustrates the generation of the frequency shifted variable duty cycle output. As seen in FIG. 6B, while the voltage of the averaged square wave signal (i.e., the output of the averaging circuit 120) is greater than the voltage of the output of the waveform generator 130, the output of the comparator 140 is set to a first voltage level, and while the value of the output of the averaging circuit 120 is below the voltage of the output of the waveform generator 130, the output of the comparator 140 is set to a second voltage level, e.g., ground (i.e., whenever the plot of the voltage of the averaging circuit crosses the plot of the output of the waveform generator to become larger than the output of the waveform generator, the output of the comparator is switched to the first voltage level, and whenever the plot of the voltage of the averaging circuit crosses the plot of the output of the waveform generator to become smaller than the output of the waveform generator, the output of the comparator is switched to the second voltage level). Thus, the output of the comparator 140 is a square wave that transitions between the first voltage level and the second voltage level (e.g., 10 V and ground), has a duty cycle that corresponds to the level of the voltage output by the averaging circuit 120 and has a frequency corresponding to the frequency of the output of the waveform generator 130. The first and second voltage levels may be any suitable voltage levels and may be selected based upon the particular LED driver circuit with which the duty cycle detection and frequency conversion circuit 100 is being utilized.

In embodiments in which the duty cycle of the duty cycle detection circuit is inversely related to the input voltage (as discussed above), while the voltage of the averaged square wave signal (i.e., the output of the averaging circuit 120) is greater than the voltage of the output of the waveform generator 130, the output of the comparator 140 is instead set to a second voltage level (e.g., ground), and while the value of the output of the averaging circuit 120 is below the voltage of the output of the waveform generator 130, the output of the comparator 140 is instead set to a first voltage level, with the result that, as with the embodiment shown in FIG. 6B, the comparator 140 is a square wave that transitions between the first voltage level and the second voltage level (e.g., 10 V and ground), has a duty cycle that corresponds to the level of the voltage output by the averaging circuit 120 and has a frequency corresponding to the frequency of the output of the waveform generator 130.

While FIG. 6B illustrates a generated waveform in the shape of a triangular sawtooth, any desired waveform shape can be employed. For example, the waveform can be of any of the shapes depicted in FIGS. 15A through 15E. FIG. 15A shows a non-linear waveform which includes linear portions 201 and curved portions 202 in a repetitive pattern. FIG. 15B shows a non-linear waveform which also includes linear portions 201 and curved portions 202 in a repetitive pattern. FIG. 15C shows a linear waveform which includes linear portions 201 and 203 which are of differing steepness (i.e., absolute value of slope). FIG. 15D shows a linear waveform which consists of a repeating pattern which includes two differently-shaped sub-portions 204 and 205. FIG. 15E shows a non-linear waveform which consists of a repeating pattern which includes tow differently-shaped sub-portions 206 and 207. It is readily seen that there are an infinite number of possible waveforms, and persons skilled in the art can readily select any desired waveform in order to achieve desired characteristics.

As can be seen from FIGS. 5A through 6B, the shape of the waveform output from the waveform generator 130 may affect the relationship between the input voltage duty cycle and the output duty cycle of the duty cycle detection and frequency conversion circuit 100. If the waveform is linear (i.e., consists of linear and/or substantially linear segments) in the range over which the voltage output by the averaging circuit 120 operates, then the relationship between input duty cycle and output duty cycle will be linear. If the waveform is non-linear in at least part of the range over which the voltage output by the averaging circuit 120 operates, then the relationship between input duty cycle and output duty cycle will be non-linear.

Likewise, offsets between the input duty cycle and the output duty cycle may be provided by a DC offset which adjusts the waveform output from the waveform generator 130 and/or the voltage level output from the averaging circuit 120. For example, in a system in which the voltage level of the averaged square wave is related to (or proportional to) the duty cycle of the input voltage, and in which the frequency shifted variable duty cycle output is a first voltage level when the voltage of the averaged square wave signal is greater than the voltage of the output of the waveform generator, if the output of the waveform generator 130 is offset such that the highest voltage level reached by the waveform is lower than the voltage output by the averaging circuit 120 with duty cycles of 90% or higher, then the output of the comparator would be a constant (DC) signal at the first voltage level except when the duty cycle of the input waveform falls below (i.e., is less than) 90%. Such variations could be made adjustable and/or selectable, for example, by a user. A variety of other relationships could be used, e.g., if the voltage level of the averaged square wave is inversely related to the duty cycle of the input voltage, and the frequency shifted variable duty cycle output is a first voltage level when the voltage of the averaged square wave signal is less than the voltage of the output of the waveform generator, the waveform generator can be offset such that the lowest voltage level reached by the waveform is higher than the voltage output by the averaging circuit with duty cycles of 90% or higher, such that the output of the comparator would likewise be a constant (DC) signal at the first voltage level except when the duty cycle of the input waveform falls below 90%.

Another representative example of an offset that can optionally be provided is a DC offset in which the voltage output by the averaging circuit is increased by a specific amount (i.e., in systems where the voltage level of the averaged square wave is related to the duty cycle of the input voltage) or decreased by a specific amount (i.e., in systems where the voltage level of the averaged square wave is inversely related to the duty cycle of the input voltage). Such an offset can be useful for a variety of purposes, e.g., to compensate for a circuit in which duty cycle detection (symmetric or asymmetric) does not use zero cross detection, such that even a 100% duty cycle rectified power signal would not produce a constant signal (i.e., where the voltage depicted in FIG. 6A would be at the first voltage level 100% of the time). In such a situation, the voltage output by the averaging circuit could be increased such that where the duty cycle of the rectified power signal is 100%, the output of the averaging circuit is representative of a 100% duty cycle power signal (even though the output of the duty cycle detection circuit generated in response to the input waveform exhibits the first voltage level only part of the time, e.g., 95% of the time (and thus the averaged square wave represents a percentage duty cycle which is higher, e.g., by 5%, than the percentage of the time that the square wave representation of AC phase cut exhibits the first voltage level).

FIG. 7 illustrates further embodiments of the present inventive subject matter where the duty cycle detection and frequency conversion circuit 200 also includes a minimum pulse width detection feature. Many triac based dimmers have performance problems at light load levels which can be present with LED based lighting products at low duty cycle dimming levels. If the triac dimmers fall below their minimum load level, their output may be unpredictable, which may result in unpredictable output from a lighting device connected to the dimmer. Likewise, if the pulse width is too small, the minimum voltage requirements of the lighting device may not be met and the power supply might be starved for power. This condition may also be undesirable. As such, the ability to shut down a power supply or lighting device before the undesirable conditions resulting from low pulse width on the line input can avoid unpredictable and undesirable performance of the lighting device. Thus, the minimum pulse width detection circuit 150 allows for setting the low level dimming point by detecting when the voltage output by the averaging circuit 120 falls below (or above, in embodiments where the duty cycle of the output of the duty cycle detection circuit is inversely related to the duty cycle of the input voltage) a threshold voltage associated with the minimum duty cycle for which the lighting device and/or dimmer will operate reliably.

FIG. 8 illustrates still further embodiments of the present inventive subject matter. As seen in FIG. 8, the duty cycle detection and frequency conversion circuit 300 includes a slope adjust circuit 160. The slope adjust circuit 160 provides a method to offset the duty cycle ratio between the duty cycle determined from the variable duty cycle waveform, such as a rectified AC line with phase cut dimming, and the PWM output provided to the LED driver circuit. This would allow for a lower light level while still maintaining a sufficient AC voltage from the triac dimmer to power a lighting device.

FIG. 9 is a circuit diagram of a duty cycle detection and frequency conversion circuit 100 according to some embodiments of the present inventive subject matter. As seen in FIG. 9, the rectified AC line voltage is scaled to appropriate voltage levels, for example, by dividing the voltage down through a resistor divider network, and sent to the positive input of a first comparator U1. The comparator U1 compares the scaled and rectified AC to a fixed voltage reference (V_(thr)) at the negative input. When the positive input exceeds the negative, the output of the comparator U1 is high; when the reverse is true, the output is low (on the other hand, in embodiments where the duty cycle of the output of the duty cycle detection circuit is inversely related to the duty cycle of the input voltage, the comparator U1 is reversed, such that the rectified AC input voltage is supplied to the negative input of the comparator U1 and the fixed voltage reference is supplied to the positive input of the comparator U1). The resultant waveform is a close representation of the non-zero voltage duty-cycle of the AC line (the closer the fixed voltage reference V_(thr) is to zero, the closer the resultant waveform approximates the non-zero voltage duty cycle of the AC line). The resultant waveform is a fixed amplitude square wave with a duty cycle and a frequency which correspond to the duty cycle and frequency of the rectified AC line. The reference voltage V_(thr) sets the maximum pulse width of the square wave output of the comparator U1. The closer the reference voltage V_(thr) is to zero volts the greater the maximum pulse width (for example, if V_(thr) is 5 V, the maximum pulse width is 100% minus the percentage of the time that the pulse is less than 5 V (the percentage of the time that the pulse is less than 5 V corresponding to the percentage of the plot, viewed along the x axis, where the plot is less than 5 V)). In some embodiments, the reference voltage may be set to a value that reduces or eliminates half cycle imbalances in a rectified triac phase cut AC waveform. Skilled artisans are familiar with ways to make the reference voltage zero (or very close to zero), e.g., by providing AC sensing detection, such as zero cross detection.

The variable duty-cycle fixed amplitude square wave from the duty cycle detection circuit 110 is then filtered by the averaging circuit 120 to create an average value; higher level for higher duty-cycles, lower level for lesser duty-cycles (the opposite is of course true in embodiments where the duty cycle of the output of the duty cycle detection circuit is inversely related to the duty cycle of the input voltage). Because the square wave is of fixed amplitude, the average value is proportional to the duty cycle of the square wave, which is proportional to the duty-cycle of the input waveform, such as the AC line input. The averaging circuit 120 is illustrated as a filter that includes resistor R1 and capacitor C1. While a single stage RC filter is illustrated in FIG. 9, other filtering or averaging techniques could be utilized. For example, in some embodiments, an RC filter with two or more stages may be used.

The output of the RC filter is provided to the positive input of a second comparator U3 and is compared to a fixed-frequency fixed-amplitude triangle/sawtooth wave generated by the op amp (i.e., operational amplifier) U2, resistors R2, R3 and R4 and the capacitor C2. The triangle/sawtooth waveform is connected to the negative input of the comparator U3 (in embodiments in which the duty cycle of the output of the duty cycle detection circuit is inversely related to the duty cycle of the input voltage, the waveform is instead connected to the positive input of the comparator U3). The output of the comparator U3 is a square wave which has a duty-cycle proportional to the voltage level at the positive input of the comparator U3 (the output of the averaging circuit 120) and a frequency equal to that of the triangle/sawtooth wave. In this manner, the duty cycle of, for example, a lower frequency AC line can be translated to a higher frequency square wave. The square wave can be used to gate LEDs on and off for a dimming effect.

FIG. 9 illustrates the use of a single op amp sawtooth generator as the waveform generator 130. Other circuits may also be utilized to generate appropriate waveforms. For example, a two op amp triangle oscillator as described on page A-44 of “Op Amps for Everyone,” R. Mancini, Editor, September 2000, may also be utilized. Other circuits known to those of skill in the art may also be used. When using a waveform generator such as illustrated in FIG. 9, to provide a linear relationship (or substantially linear relationship) between input and output duty cycle, the portions of the resulting waveform for the range over which the average value voltage will vary should be linear (or substantially linear). For example, the waveform generator illustrated in FIG. 9 may provide a waveform with a linear region and a non-linear region that resembles a “shark fin.” If the range of voltages output by the averaging circuit 120 overlaps with the non-linear region, then a small change in input duty cycle could result in a large change in output duty cycle, or vice-versa. Such a situation may make the overall circuit susceptible to noise or too sensitive to variations in input duty cycle (e.g. too sensitive to user input at a dimmer). As a result, the circuit illustrated in FIG. 9 may be implemented such that the voltage range of the averaging circuit 120 corresponds to a linear portion or portions of the output waveform from the waveform generator 130.

FIG. 10 is a circuit diagram of a duty cycle detection and frequency conversion circuit 100′ that provides asymmetric threshold voltages for duty cycle detection. As seen in FIG. 10, the duty cycle detection circuit 110′ includes a second comparator U4, a logic AND gate A1 and a Set/Reset latch L1 that provide independently settable on and off thresholds. As discussed above, the triac based AC waveform can have half cycle imbalances that the voltage threshold(s) critical may be set based upon to provide steady PWM duty cycle generation.

In operation, the duty cycle detection circuit 110′ sets the latch L1 when the input voltage becomes higher than the threshold voltage V₁ and resets the latch L1 when the input voltage falls below the threshold voltage V₂, where V₁>V₂. In particular, when the input voltage exceeds V₁, the output of the comparator U1 is high and the set input S of the latch L1 is high so as to cause the output Q of the latch L1 to go high. When the input voltage falls below V₁, the output of the comparator U1 goes low but the output Q of the latch L1 remains high. When the input further falls below V₂, the output of the comparator U4 goes high, therefore both inputs to the AND gate A1 are high so the output of the AND gate A1 goes high, resetting the latch L1, and the output Q goes low. While the circuit illustrated in FIG. 10 has been designed for V₁>V₂, a corresponding circuit where V₁<V₂ could be readily provided by logically ANDing the inverted output of the latch L1 with the output of comparator U1 and using the output of the AND as the set signal for the latch L1. In such a case, the AND gate A1 could be eliminated and the output of the comparator U4 provided directly to the rest of the latch L1.

FIG. 11 is a circuit diagram illustrating a duty cycle detection and frequency conversion circuit 200 that incorporates a minimum pulse width detection circuit 150. As seen in FIG. 11, the minimum pulse width detection circuit 150 is provided by the comparator U5. In particular, a reference voltage V_(shut) is provided to one input of the comparator U5 and the output of the averaging circuit 120 is provided to the other input. In this embodiment, the output of the averaging circuit is related to the output of the duty cycle detection circuit. When the output of the averaging circuit falls below the reference voltage V_(shut), the output of the comparator U5 goes high, thus providing a shutdown signal. In alternative embodiments, in which the output of the averaging circuit is inversely related to the output of the duty cycle detection circuit, the output of the comparator U5 goes high to provide a shutdown signal when the output of the averaging circuit rises above the reference voltage V_(shut).

FIG. 12 is a circuit diagram of a duty cycle detection circuit 100 coupled to an LED driver circuit where the string of LEDs (LED1, LED2 and LED3) is driven by an input voltage that is modulated by a high frequency drive signal through the transistor T1. The diode D1, capacitor C3 and inductor L1 provide current smoothing between cycles of the high frequency drive signal. The resistor R5 provides a current sense that can be fed back to a driver controller that varies the duty cycle of the high frequency drive signal to provide constant current to the LEDs. The gate of the transistor T1 is controlled by the driver DR1. The driver is enabled by the output of the duty cycle detection and frequency conversion circuit 100 so that the high frequency drive signal is controlled by the output of the duty cycle detection and frequency conversion circuit 100. Because the transistor T1 is controlled by the output of the duty cycle detection and frequency conversion circuit 100, it may be necessary to disable or otherwise control or compensate for the current sense feedback to the controller when the transistor T1 is off, as the sensed current feedback is only valid when the transistor T1 is on.

FIGS. 13 and 14 are flowchart illustrations of operations according to some embodiments of the present inventive subject matter. It will be appreciated that the operations illustrated in FIGS. 13 and 14 may be carried out simultaneously or in different sequences without departing from the teachings of the present inventive subject matter. Thus, embodiments of the present inventive subject matter should not be construed as limited to the particular sequence of operations illustrated by the flowcharts. Furthermore, operations illustrated in the flowcharts may be carried out entirely in hardware or in combinations of hardware and software.

Turning to FIG. 13, the duty cycle of the input waveform is detected to provide a fixed amplitude duty cycle signal (block 500). The average is determined of the fixed amplitude signal to generate an average value which may be reflected as a voltage level (block 510). A waveform of a different frequency from the frequency of the input signal is generated (block 520) and the value of the waveform is compared to the average value (voltage level) to generate a waveform with a duty cycle corresponding to (i.e., not necessarily the same as, but “based on”) the input duty cycle at a frequency corresponding to the frequency of the generated waveform (block 530).

FIG. 14 illustrates further operations according to some embodiments of the present inventive subject matter. As seen in FIG. 14, the duty cycle of the input waveform is detected to provide a fixed amplitude signal with a duty cycle corresponding to the duty cycle of the input waveform (block 600). The average value of the fixed amplitude signal is determined to generate an averaged voltage corresponding to the average value of the fixed amplitude signal (block 610). The averaged voltage level is compared to a voltage level for the minimum pulse width to determine if the pulse width of the input signal is less than the minimum allowable pulse width (block 620). If the averaged voltage level is below this level (block 620), the shutdown signal is provided (block 670). If the averaged voltage level is above the minimum allowable pulse width level (block 620), the averaged voltage level is compared to the voltage of a generated waveform (block 640). The generated waveform is of a frequency different from that of the input signal (block 630). If the averaged voltage level is above the voltage of the generated waveform (block 640), a high signal is output (block 660). If the averaged voltage is below the voltage of the generated waveform (block 640), a low signal is output (block 650).

FIGS. 16A-16F are circuit diagrams depicting an embodiment of a circuit according to the present inventive subject matter. FIG. 16A depicts a lighting control circuit including a duty cycle detection circuit 110, an AC scaling circuit 115, a power source 116, an averaging circuit 120, a waveform generator 130, a comparator 140 and a minimum pulse width detection circuit 150. FIG. 16B is a blown-up view of the duty cycle detection circuit 110, the AC scaling circuit 115 and the power source 116. FIG. 16C is a blown-up view of the averaging circuit 120. FIG. 16D is a blown-up view of the waveform generator 130. FIG. 16E is a blown-up view of the comparator 140. FIG. 16F is a blown-up view of the minimum pulse width detection circuit 150.

The generation of a square wave representation of an input waveform duty cycle, such as the AC line voltage, in this manner is tolerant of variations in line voltage and frequency, i.e. the square wave will remain the same even if the AC line voltage or frequency increases or decreases due to utility generation, load adding or shedding, or other reasons. A circuit which, unlike the present invention, filters the rectified line would be unable to differentiate between changes in duty cycle and changes in line voltage, and the representative filtered level would change in response—the present inventive subject matter overcomes these drawbacks.

The generated waveform used as the comparison source for the final output may be altered in frequency or shape. Altering the shape of the generated waveform can change the proportional response of the output to the AC input, e.g., if desired, to create a highly non-linear dimming response to the AC input.

The higher frequency output, used as a manner to switch on and off the LEDs, can eliminate human visible flicker, and/or the flicker as recorded by electronics such as video cameras.

Using the methods and circuits according to the present inventive subject matter, a light or a set of lights connected to a driver as described herein can be connected to a power source, through a circuit in accordance with the present inventive subject matter, without concern as to the frequency of the voltage from the power source and/or the voltage level of the power source. To illustrate, skilled artisans are familiar with a variety of situations in which the frequency of the line voltage is 50 Hz, 60 Hz, 100 Hz or other values (e.g., if connected to a generator, etc.) and/or in which the line voltage can change or vary, and the problems that can be caused, particularly with conventional dimmers, when connecting a light or set of lights to such line voltage. With circuitry as described herein, a light or set of lights can be connected to line voltages of widely differing frequencies and/or which vary in voltage level, with good results.

In addition, the present inventive subject matter has been described with regard to dimming, but the present inventive subject matter is also applicable to modifying other aspects of the light output, e.g., color temperature, color, hue, brightness, characteristics of the outputs of the light, CRI Ra, etc. For example, a lighting control circuit can be configured such that when the duty cycle of the input voltage is a certain percentage (e.g., 10%), the circuitry can cause the output of the device to have a particular color temperature (e.g., 2,000 K). For instance, with natural light, as the light dims, the color temperature typically decreases, and it might be deemed desirable for the lighting device to mimic this behavior. In addition, with security lighting, it can be desirable for dimmed lighting to have low CRI, such that there is enough light that an intruder can be observed, but the CRI Ra is low enough that the intruder has difficulty seeing what he or she is doing.

The circuits and methods according to the present inventive subject matter are not limited to AC power or to AC phase cut dimmers. Rather, the present inventive subject matter is applicable to all types of dimming using waveform duty cycle (e.g., including pulse width modulation).

While certain embodiments of the present inventive subject matter have been illustrated with reference to specific combinations of elements, various other combinations may also be provided without departing from the teachings of the present inventive subject matter. Thus, the present inventive subject matter should not be construed as being limited to the particular exemplary embodiments described herein and illustrated in the Figures, but may also encompass combinations of elements of the various illustrated embodiments.

Many alterations and modifications may be made by those having ordinary skill in the art, given the benefit of the present disclosure, without departing from the spirit and scope of the inventive subject matter. Therefore, it must be understood that the illustrated embodiments have been set forth only for the purposes of example, and that it should not be taken as limiting the inventive subject matter as defined by the following claims. The following claims are, therefore, to be read to include not only the combination of elements which are literally set forth but all equivalent elements for performing substantially the same function in substantially the same way to obtain substantially the same result. The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, and also what incorporates the essential idea of the inventive subject matter.

Any two or more structural parts of the devices described herein can be integrated. Any structural part of the devices described herein can be provided in two or more parts (which are held together, if necessary). Similarly, any two or more functions can be conducted simultaneously, and/or any function can be conducted in a series of steps. 

1. A lighting control circuit comprising: a duty cycle detection circuit configured to generate a first periodic waveform having a detection circuit duty cycle and detection circuit frequency corresponding to an input waveform duty cycle and input waveform frequency; an averaging circuit responsive to the duty cycle detection circuit and configured to generate a first signal having a voltage level corresponding to the detection circuit duty cycle; a waveform generator configured to output a second periodic waveform having a waveform generator frequency different from the input waveform frequency; and a comparator circuit configured to compare the second periodic waveform with the first signal to generate a comparator waveform having a comparator circuit duty cycle corresponding to the input waveform duty cycle and a comparator circuit frequency corresponding to the waveform generator frequency.
 2. A lighting control circuit as recited in claim 1, wherein the first signal has a voltage level which is related to the detection circuit duty cycle.
 3. A lighting control circuit as recited in claim 2, wherein the lighting control circuit further comprises a shutdown comparator circuit which is configured to compare the first signal with a shutdown threshold voltage and to generate a shutdown signal if the first signal falls below the shutdown threshold voltage.
 4. A lighting control circuit as recited in claim 1, wherein the first signal has a voltage level which is inversely related to the detection circuit duty cycle.
 5. A lighting control circuit as recited in claim 4, wherein the lighting control circuit further comprises a shutdown comparator circuit which is configured to compare the first signal with a shutdown threshold voltage and to generate a shutdown signal if the first signal rises above the shutdown threshold voltage.
 6. A lighting control circuit as recited in claim 1, wherein the duty cycle detection circuit is configured to output a first voltage level while the voltage of the input waveform is above an input threshold level, and to output a second voltage level while the voltage of the input waveform is below the input threshold level.
 7. A lighting control circuit as recited in claim 1, wherein the duty cycle detection circuit is configured to output a first voltage level when the voltage of the input waveform is above a first input threshold level, to continue to output the first voltage level after the voltage of the input waveform rises above the first input threshold level until the voltage of the input waveform falls below a second input threshold level, to output a second voltage level when the voltage of the input waveform falls below the second input threshold level, and to continue to output the second voltage level after the voltage of the input waveform falls below the first input threshold level until the voltage of the input waveform rises above the first input threshold level.
 8. A lighting control circuit as recited in claim 1, wherein the comparator circuit duty cycle is linearly related to the input waveform duty cycle.
 9. A lighting control circuit as recited in claim 1, wherein the comparator circuit duty cycle is non-linearly related to the input waveform duty cycle.
 10. A lighting control circuit as recited in claim 1, wherein the voltage level of the first signal is independent of an RMS voltage of the input waveform for a predetermined range of RMS voltage values.
 11. A lighting device comprising: at least one solid state light emitter; a lighting control circuit as recited in claim 1; and a driver circuit configured to vary the intensity of output of the at least one solid state light emitter in response to the comparator waveform.
 12. A lighting control circuit comprising: means for generating a first periodic waveform having a first waveform duty cycle and first waveform frequency corresponding to an input waveform duty cycle and input waveform frequency; means for generating a first signal having a voltage level corresponding to the first waveform duty cycle; means for outputting a second periodic waveform having a second waveform frequency different from the input waveform frequency; and means for comparing the second periodic waveform with the first signal to generate a comparison waveform having a comparison waveform duty cycle corresponding to the input waveform duty cycle and a comparison waveform frequency corresponding to the second waveform frequency.
 13. A lighting control circuit as recited in claim 12, wherein the first signal has a voltage level which is related to the first waveform duty cycle.
 14. A lighting control circuit as recited in claim 13, wherein the lighting control circuit further comprises means for comparing the first signal with a shutdown threshold voltage and generating a shutdown signal if the first signal falls below the shutdown threshold voltage.
 15. A lighting control circuit as recited in claim 12, wherein the first signal has a voltage level which is inversely related to the first waveform duty cycle.
 16. A lighting control circuit as recited in claim 15, wherein the lighting control circuit further comprises means for comparing the first signal with a shutdown threshold voltage and generating a shutdown signal if the first signal rises above the shutdown threshold voltage.
 17. A lighting control circuit as recited in claim 12, wherein the means for generating a first periodic waveform outputs a first voltage level while the voltage of the input waveform is above an input threshold level, and outputs a second voltage level while the voltage of the input waveform is below the input threshold level.
 18. A lighting control circuit as recited in claim 12, wherein the means for generating a first periodic waveform outputs a first voltage level when the voltage of the input waveform is above a first input threshold level, continues to output the first voltage level after the voltage of the input waveform rises above the first input threshold level until the voltage of the input waveform falls below a second input threshold level, outputs a second voltage level when the voltage of the input waveform falls below the second input threshold level, and continues to output the second voltage level after the voltage of the input waveform falls below the first input threshold level until the voltage of the input waveform rises above the first input threshold level.
 19. A lighting control circuit as recited in claim 12, wherein the comparison waveform duty cycle is linearly related to the input waveform duty cycle.
 20. A lighting control circuit as recited in claim 12, wherein the comparison waveform duty cycle is non-linearly related to the input waveform duty cycle.
 21. A lighting control circuit as recited in claim 12, wherein the voltage level of the first signal is independent of an RMS voltage of the input waveform for a predetermined range of RMS voltage values.
 22. A lighting device comprising: at least one solid state light emitter; a lighting control circuit as recited in claim 12; and means for varying the intensity of output of the at least one solid state light emitter in response to the comparison waveform.
 23. A method of controlling lighting, comprising: generating a first periodic waveform having a first waveform duty cycle and first waveform frequency corresponding to an input waveform duty cycle and input waveform frequency; generating a first signal having a voltage level corresponding to the first waveform duty cycle; outputting a second periodic waveform having a second waveform frequency different from the input waveform frequency; and comparing the second periodic waveform with the first signal to generate a comparison waveform having a comparison waveform duty cycle corresponding to the input waveform duty cycle and a comparison waveform frequency corresponding to the second waveform frequency.
 24. A method as recited in claim 23, wherein the first signal has a voltage level which is related to the first waveform duty cycle.
 25. A method as recited in claim 24, wherein the method further comprises comparing the first signal with a shutdown threshold voltage and generating a shutdown signal if the first signal falls below the shutdown threshold voltage.
 26. A method as recited in claim 23, wherein the first signal has a voltage level which is inversely related to the first waveform duty cycle.
 27. A method as recited in claim 26, wherein the method further comprises comparing the first signal with a shutdown threshold voltage and generating a shutdown signal if the first signal rises above the shutdown threshold voltage.
 28. A method as recited in claim 23, wherein the first periodic waveform has a first voltage level while the voltage of the input waveform is above an input threshold level, and has a second voltage level while the voltage of the input waveform is below the input threshold level.
 29. A method as recited in claim 23, wherein the first periodic waveform has a first voltage level when the voltage of the input waveform is above a first input threshold level, continues to have the first voltage level after the voltage of the input waveform rises above the first input threshold level until the voltage of the input waveform falls below a second input threshold level, has a second voltage level when the voltage of the input waveform falls below the second input threshold level, and continues to have the second voltage level after the voltage of the input waveform falls below the first input threshold level until the voltage of the input waveform rises above the first input threshold level.
 30. A method as recited in claim 23, wherein the comparison waveform duty cycle is linearly related to the input waveform duty cycle.
 31. A method as recited in claim 23, wherein the comparison waveform duty cycle is non-linearly related to the input waveform duty cycle.
 32. A method as recited in claim 23, wherein the voltage level of the first signal is independent of an RMS voltage of the input waveform for a predetermined range of RMS voltage values. 